The file contains all details of the Feedwater used and the treatment applied on it before using in the Thermal power plant. This is the part of the subject Power Plant Engineering in GTU in 7th semester.

1. Feed Water Treatment
Faculty Name: Prof. Ajaypalsinh G. Barad
Branch: Mechanical Engineering
College : S. P. C. E., Visnagar
Semester: 7th Semester
Name of Subject: Power Plant Engineering

2. Outline
• Necessity of feed water treatment
• Different impurities found in feed water
• Effect of impurities
• pH & its role in corrosion and scale formation
• Internal & external water treatment systems
• Demineralization plants
• Reverse osmosis process
• Sea water treatment using reverse osmosis
• De-aeration
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3. Necessity of feed water treatment:
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• Its your turn to speak out.

4. Different impurities found in feed water
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1. Undissolved and suspended materials
a) Turbidity and sediment
b) Sodium and Potassium salts
c) Chlorides
d) Iron
e) Manganese
f) Silica
g) Microbiological growths
h) Colour

5. Different impurities found in feed water
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2. Dissolved salts and Minerals
a) Calcium and Magnesium salts
3. Dissolved Gases
a) Oxygen
b) Carbon Dioxide
4. Other materials
a) Free mineral acid
b) Oil

6. Effect of impurities
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1. Scale Formation

7. Effect of impurities
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2. Corrosion
– Corrosion is the destructive conversion of metal into oxides
or salts. It causes wearing of the boiler metal and produces
pits, grooves and cracks on the boiler surface.
– Corrosion on inner surface of boiler tubes is mainly caused
due to presence dissolved O2 in boiler feed water.
– Carbon dioxide is next to oxygen which is responsible for
corrosion. Dissolved CO2 in water forms carbonic acid.
• CO2 + H2O —> H2CO3
– Carbonic acid reacts with iron and other metals to form
bicarbonates. They are decomposed by heat and CO2 is
again liberated. CO2 again form carbonic acid.

8. Effect of impurities
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3. Priming, Foaming and Carryover
• Due to presence of soluble and insoluble salts in
water, steam passage is filled with mass of frothy
bubbles. This is known as foaming.
• Foaming can be reduced by adding castor oil,
antifoaming agents and by adding sodium aluminate.
• During rapid steam production, some water droplets
are also carried along with the steam as it leaves the
boiler. This wet steam formation is called priming.

9. Effect of impurities
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3. Priming, Foaming and Carryover
• Priming and foaming occurs together and is referred
as carry over. Following precautions should be
observed to reduce foaming and priming:
o Oil, soap and other suspended impurities should be
removed from boiling feed water.
o Various valves should not be opened suddenly to
maximum.
o Water in the boiler should be at its maximum possible
level.

10. Effect of impurities
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4. Caustic Embrittlement
• Caused due to presence of caustic impurities present
in water. Presence of certain concentration of NaOH
and silica causes embrittlement.
• Presence of NaOH makes the metal brittle and inner
cracks appear along the seams below the water level.
NaOH flows into minute hair cracks of the boiler surface
by capillary action.
• Can be prevented by adding sodium phosphate as
softening agent, addition of organic agents like tannin or
lignin and addition of Na2SO4 to water.

11. Effect of impurities
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4. Caustic Embrittlement

12. pH & its role in corrosion and scale formation
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• pH value of water is the logarithm of the reciprocal of
hydrogen (H+) ion concentration in water. If the
concentration of H+ ion in water is 10-7, then the pH
value of water will be:
• pH value varies from 0 to 14. A sample of water having
pH value less than 7 indicates acidity of water and more
than 7 indicated alkalinity of water.
• As the pH is a logarithmic function, solution having pH
value of 6,5 and 4 are respectively 10, 100 and 1000 times
more acidic than the solution having pH value of 7.

13. pH & its role in corrosion and scale formation
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Role of pH :
• The corrosion rate of iron in absence of oxygen is
proportional to pH value. At lower pH value, iron will
be more soluble.
• The oxygen in the water unites with ferrous hydroxide
to form ferric hydroxide. The pH value of feed water
should be kept greater than 9.6.
• Corrosion rate increases due to presence of dissolved
O2 and CO2 in water as concentration of H+ ions
increases which increases the pH value of water.

14. Internal water treatment systems
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• In this method, dissolved solids are removed in the
boiler itself, by adding chemicals either to precipitate
the impurities so that they can be removed in the form
of sludge or to convert impurities into harmless salts.
• Various methods of internal treatment are:
1. Sodium carbonate treatment
2. Phosphate treatment
3. Sodium aluminate
4. Colloidal treatment
5. Blow down system

15. Sodium carbonate treatment
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• Dissolved solids are removed by adding Na2CO3
(Soda Ash) to boiler water.

16. Phosphate treatment
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• Scale formation avoided by adding sodium phosphate.

17. Sodium aluminate
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• The scale formation avoided by adding NaAlO2.

18. Colloidal treatment
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• It is used in low pressure boilers to prevent scale
formation by adding organic substances like tannin,
starches, lignin, kerosene.
• Coating of these substances is formed over scale
forming precipitates.
• It results in formation of non-sticky and loose
deposits similar to sludge which can be removed by
blow down.

19. Blow down system
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20. Blow down system
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21. Blow down system
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22. External water treatment systems
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• When the quantity of make up water is large and it
contains considerable suspended and dissolved
impurities, then external water treatment is used. In
this method of treatment, raw water is taken to a tank
where reagents are mixed.
• Various methods:
1. Mechanical
 Sedimentation
 Coagulation
 Filtration

23. External water treatment systems
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2. Thermal
 Distillation (Evaporation)Sedimentation
 Deaeration
3. Chemical
 Lime-Soda treatment
 Zeolite process
 Demineralization

24. Sedimentation
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• In this method, water is allowed to remain stand-still in
big tanks or allowed to flow at very low velocity.
• The solid matter settles down due to gravity which is then
removed periodically or continuously.
• Clean water is taken from the surface of the tank.

25. Coagulation :
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• In this method, coagulants like aluminum sulphate,
sodium aluminate or ferrous sulphate are added to the
impure water.
• Reaction between these salts and alkalinity in the water
causes smaller particles to adhere to each other, forming
larger particles.
• These larger particles settle down and can be removed by a
filtration process.

26. Coagulation :
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27. Filtration :
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• In this method, water is allowed to pass through gravity or
pressure filters.
• By increasing the fineness of the filtering element,
maximum suspended impurities are removed.
• The suspended matter adheres to the filter material and
clear water drains from the bottom.
• It is necessary to clean the dirt collected in the voids of the
filter material periodically by backwashing the filter
material.

28. Filtration :
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29. Distillation (Evaporation) :
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30. Distillation (Evaporation) :
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31. Distillation (Evaporation) :
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• In this method, raw water is evaporated utilizing bled
steam from the boiler.
• The steam from boiler flows into the tubes and gets
condensed after transferring the heat to the water as shown
in figure.
• The condensate is returned to the boiler as feed water.
• The water from top surface gets evaporated. The steam is
collected and condensed to supply pure feed water.

32. Deaeration :
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33. Hot lime soda process
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34. Hot lime soda process
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35. Hot lime soda process
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• Chemical reaction take place are listed below:
• The carbonate hardness is removed by hot lime,
• The sulphate hardness of calcium and magnesium is
removed by soda ash,

36. Hot lime soda process
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37. Hot lime soda process
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• Advantages:
– Process is economical
– It is capable of treating large quantities of water in a
relatively small unit.
– pH of the water rises which improves the corrosion
resistance of water.
– Silica is effectively removed from the water.
– The size of the settling tank is reduced as the
precipitate is larger in size and settles more quickly.

38. Hot lime soda process
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• Disadvantages:
– Softness produced by this method is less than the
zeolite process.
– Huge amount of sludge is formed and disposal is
difficult.
– Controlling of hot process is difficult.
– The softened water must be filtered to avoid
precipitates being carried with softened water.
– Back washing of filter periodically is required to
increase the efficiency of the filters.

39. Zeolite ion exchange process
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40. Zeolite ion exchange process
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• Advantages
– Hardness of water can be removed almost completely
up to 10 ppm.
– There is no sludge formation, hence the process is clean
– The process automatically adjust itself for different
hardness of incoming water.
– The equipment used is small and easy to handle.
– It requires less time for softening.
– It is not necessary to filter the soft water as it is
required in lime soda process.

41. Zeolite ion exchange process
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• Disadvantages
– Turbid water cannot be admitted to the Zeolite bed
as it makes the bed inactive. Hence suspended
impurities must be filtered before passing over the
bed.
– Water with high or low pH damages the bed of
Zeolite.
– On removal of Ca+2 and Mg+2 ions, the soft water
contains large amount of NaHCO3 which on
heating, liberates CO2. this causes corrosion in the
boilers.

42. Demineralization plants
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43. Demineralization plants
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• The exhausted cation resin can be reactivated using
H2SO4 or HCl whereas the anion resin can be
reactivated using NaOH or Na2CO3.
• Advantages:
– High quantity pure distill water in the range of 0 to 2
ppm can be obtained.
– Water obtained is free from dissolved gases and silica.
• Disadvantages:
– Equipment is costly and operating cost is also high.

44. Reverse osmosis process
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45. Reverse osmosis process
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• Advantages:
– It is a efficient method of removing salts, chemical
contaminants and heavy metals such as lead from drinking
water.
– It is cheaper than ion exchange process for water containing
dissolved solids of 200 ppm or more.
– It is simple to operate and maintain.
• Disadvantages:
– Membrane must be cleaned at regular intervals.
– Membrane must be replaced every 1 to 3 years depending
on water quality and size of the system.
– Pump used requires regular maintenance.

46. Sea water treatment using reverse osmosis
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47. Deaeration :
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